1. Field of the Invention
The present invention relates to a process for the catalytic cracking of paraffinic feed streams to optimize the production of lower olefins and particularly the production of propylene.
2. Description of Related Art
Historically, light straight run naphtha (LSRN) from crude processing units was cracked in a fluidized catalytic cracking (FCC) unit. Heavy naphthas were used as reformer feedstocks to produce aromatic gasoline, a process that is still in practice today. Amorphous catalysts and dense phase cracking were part of FCC operations. The LSRN was converted into gases, gasoline and coke. Conversion of LSRN was in the range of 30% to 50%, depending upon the operating conditions. Currently, over 99% of all existing FCC units are based on riser cracking processes, which are typically ineffective for cracking of paraffinic naphtha streams.
Catalytic cracking of olefinic naphthas is well known and is currently practiced in all types of FCC units. Recycled cracked naphtha and olefinic naphthas from FCC units, visbreakers or cokers are easily converted to propylene in the FCC reactor riser with the base feedstock. In this process, the gasoline produced from recycling is high in octane and aromatics.
However, none of the current commercial FCC processes can crack LSRN efficiently and effectively to produce an increased proportion of the lower olefins and gasoline. As used herein, “lower olefins” means ethylene, propylene and butylenes.
It would therefore be desirable to provide a process in which a paraffinic naphtha feed stream is cracked to provide a light olefin product stream, and particularly one having a high propylene content. The paraffinic naphtha feed streams can be derived from a crude oil atmospheric distillation unit, or toppers, that are by-product streams from the recovery of natural gas, or from hydrotreater and hydrocracker units, or other high paraffinic naphtha streams from an extraction process, or from any other refinery or petrochemical process.